(1) Field of the Invention
The present invention relates to a process for fluidization, and more particularly, to a process for fluidization of materials which are difficult to fluidize.
(2) Description of the Prior Art
Various types of fluidization processes have been used for many years for a number of different unit operations and/or unit processes, including chemical reactions and drying operations. In the usual fluidized system, a solid phase is suspended in an upwardly moving fluid stream, usually a gas stream, whereby the mass of solid particles have the appearance of a boiling liquid. The solid phase may be a catalyst to promote a chemical reaction, with the reactants being contained in the fluidizing gas, or the solid phase may be a material which is reactive with the fluidizing gas. Alternatively, the solid phase may be a material which is treated by the fluidizing gas as in the case of fluidized drying.
One of the primary advantages of fluidized bed systems resides in the fact that the high turbulence existing in a fluidized bed provides high heat transfer characteristics. In addition, that turbulence in the fluid bed causes complete mixing of the solids with the fluidizing gas to form a relatively homogeneous gas-solid system.
Fluidized bed systems are, however, not without some disadvantages. As is now well known to those skilled in the art, the use of fluidized systems frequently results in channeling, a phenomenon caused by the formation of pockets in the solid phase which in turn results in the passage of gas through the solids forming the bed without intimate contact with the solid phase.
The problem in channeling in a fluidized bed system can be partially minimized by the use of a plurality of tubular zones through which the fluidizing gas is passed in contact with the solid phase. Each tube thus operates as an individual fluidized bed having a much smaller cross sectional area. Such tubular bed systems have even greater heat transfer characteristics because the plurality of tubular zones increase the surface available for heat transfer.
However, the use of a plurality of tubular zones, has not found acceptance in the fluidization of materials which, because of their cohesive characteristics, tend to form aggregates and are consequently difficult to fluidize. The difficulty in fluidizing such materials has been explored by Gelhart in "Types Of Gas Fluidization", Powder Technology, 7, pp. 285-292 (1973). In that publication, the author classifies solids into groups A through D, inclusive, characterizing materials having a small means size and/or a particle density less than 1.4 g/cm.sup.3 as group A materials. Group B materials are described as having a means size ranging from 40 .mu.m to 500 .mu.m and a density ranging from 1.4 to 4 g/cm.sup.3. Materials of the groups A and B do not present unusual problems from the standpoint of fluidization. Groups C and D, on the other hand, present the most severe fluidization problems, the group C materials being cohesive and as a result, tending to plug small diameter tubes.
Gelhart points out that fluidization of such materials can be made possible or improved by the use of mechanical stirrers or vibrators to minimize channeling in the fluid bed. However, the author points out that one of the more effective means to avoid difficulties with such materials is the addition of extraneous solids to the system.
There are a number of solid materials which fall into the groups C and D categories as outlined above. Starch is one example of a group C material since starch tends to be quite cohesive, and thus tends to plug small diameter tubes. Attempts have been made in the prior art to process starches in a fluidized bed system. For example, in U.S. Pat. No. 2,845,368, there is described a process for the conversion of starch to dextrin in a fluidized bed system in which the fluidized reactor includes a plurality of heat transfer tubes contained in the reactor to supply heat to the starch undergoing conversion. One of the primary difficulties with a system of the type described in the foregoing patent is that the starch, when contacted with an acid catalyst, tends to form lumps or agglomerates within the fluidized bed reactor to an even greater extent. Thus, the inherent cohesiveness of starch coupled with the increased tendency for starch to agglomerate when contacted with a catalyst results in severe channeling. Channeling, in turn, results in incomplete conversion of the starch to dextrin.
In addition, reactors used in the dextrinization of starch are frequently characterized by a "dead zone" at the upper portion of the reactor where the starch may lay and be subjected to high temperatures for extended periods. Auto ignition can occur, causing fire and/or explosions. This problem can be particularly aggravated in apparatus of the type taught by the foregoing patent for the heat transfer surfaces present in the fluidized bed reactor, when present in sufficient surface area to provide the necessary heat exchange, disrupts the fluid flow within the reactor to cause the formation of such "dead zones".